The grass and water ones were especially hard for me because I like most of the grass type starters equally, and I almost always choose water types and love all of them. The fire type one was easy though because Typhlosion is my favorite Pokemon

Grass: chikorita family
Fire: charmander family
Water: totodile family

The water type was very close between totodile and mudkip, but i chose the totodile family over the mudkip family because even though i like totodile just as much as mudkip, and croconaw just as much as marshtomp, i just slightly rather feraligator over swampert